The one shot zero is harder to do with irons but it can be done with a helper. I'll describe the process with a scope, just transfer that description to your irons. The only difference is you"ll probably have to place a spotting disc in your bullet hole to see it with irons.
Get sand bags for support because you will not want the rifle to move at all. Set your target at fifty yards and make sure you have a very large piece of backing paper in case you're not on target.
Remove your bolt/bolt carrier and place the bullseye center bore. Try to adjust the sights so they are center target while the bore shows on target too. You can remove the upper on an AR-15 to make this easier.
With sand bag support up and down for elevation, left and right for windage, completely stable and supported with the crosshairs dead on target squeaze off the very best shot you can. Now with a spotter mark where the bullet hit.
Now place the crosshairs dead center on the target again. With a helper (which is best) or without, move your sights windage first so it is perfectly aligned with your shot, make sure you do not move the rifle in the process. Now adjust your elevation the sam way, making sure you do not move the rifle. The rifle must remain in the exact position it was in when you made the initial shot.
You now have a fifty yard zero. Fifty yards is so close that it will probably not have to be adjusted at all unless you moved the rifle a little while adjusting the sights. From there you will be on paper to confirm and refine your zeroes at 100/200 yards.
